During the present study, the methanol and chloroform-soluble extracts of G. mangostana
were both found to be strongly inhibitory against aromatase in the microsomal assay (, ). A small library of 12 pure xanthones (1–12
), isolated from G. mangostana
were tested for aromatase inhibition in microsomes. Compounds were arbitrarily designated as strongly active if their percent control activity (PCA) was 0 – 10, moderately active if their PCA was >10 – 30, weakly active if their PCA was 30 – 50, and inactive if their PCA was greater than 50. Active compounds were then subjected to IC50
testing to determine if they acted in a dose-dependent manner (). Two xanthones, γ-mangostin (9
, 4.7 PCA, IC50
6.9 µM) and garcinone D (3
, 10.0 PCA, IC50
5.2 µM), were found to be strongly active in microsomes (, ) . Two other xanthones, α-mangostin (8
, 22.2 PCA, IC50
20.7 µM) and garcinone E (4
, 23.9 PCA, IC50
25.1 µM), were found to be moderately active in this microsomal assay. All other xanthones tested (1, 2, 5–7
, and 10–12
) were inactive.
Percent Control Activity (PCA) Values for Non-cellular, Enzyme-based and SK-BR-3 Cell-based Aromatase Bioassays and IC50 Values for the Non-cellular, Enzyme-based Bioassay for Xanthones 1–12.
Figure 1 Percent control activity (PCA) of extracts (at 20 µg/mL) and compounds (at 20 µg/mL) from mangosteen (G. mangostana) tested in a noncellular, enzyme-based, microsomal aromatase bioassay (DMSO = dimethylsulfoxide, blank/negative control; (more ...)
Figure 2 IC50 curves for active compounds from mangosteen: (A) garcinone D (3, IC50 = 5.16 µM), (B) garcinone E (4, IC50 = 25.14 µM), (C) α-mangostin (8, IC50 = 20.66 µM), and (D) γ-mangostin (9, IC50 = 6.88 µM). (more ...)
To determine if the G. mangostana
MeOH and CHCl3
extracts and compounds 3, 4, 8
, and 9
inhibited aromatase in a more biologically relevant, cell-based assay, these samples were then tested at 50 µM in a secondary cell-based assay, using SK-BR-3 human breast cancer cells that overexpress the aromatase enzyme. Inhibitory activity was evident for both of the two mangosteen extracts, with γ-mangostin (9
) being the only one of the four compounds found to strongly inhibit aromatase in cells (−0.5 PCA), garcinone E (4
) being moderately inhibitory (32.3 PCA), and garcinone D (3
) and α-mangostin less so (> 50 PCA) (). However, γ-mangostin (9
) was also found to be fairly cytotoxic in SK-BR-3 cells (), complicating the determination if the aromatase inhibition was due to actual activity or the result of low cell survival. Therefore, γ-mangostin (9
) was further subjected to IC50
testing in both the SK-BR-3 cell-based aromatase assay and SK-BR-3 cell-based cytotoxicity assay (). The IC50
of γ-mangostin (9
) in the cell-based AI assay was determined to be 4.97 ± 1.9 µM, while the IC50
in the cell-based cytotoxicity assay was found to be 25.99 ± 1.0 µM. Using the concept of a chemopreventive index (CI), which provides an indication of the therapeutic index and can be computed using the equation CI = cytotoxicity IC50
/aromatase inhibition IC50
the CI for γ-mangostin (9
) was calculated as 5.2, thus indicating over five times stronger inhibition of aromatase than cytotoxicity. As such, γ-mangostin (9
) and the botanical dietary supplement mangosteen show considerable activity as aromatase inhibitors. Further molecular and in vivo studies should be performed to advance their development in this regard.
Percent cell survival of extracts and compounds from mangosteen (G. mangostana) tested in a SK-BR-3 cell-based cytotoxicity bioassay (DMSO = dimethylsulfoxide, blank/negative control).
IC50 curves for γ-mangostin (9) in (A) SK-BR-3 aromatase bioassay (IC50 = 4.97 µM), and (B) SK-BR-3 cytotoxicity bioassay (IC50 = 25.99 µM).
Xanthones 3, 4, 8
, and 9
are among the most potent natural products known to date in the microsomal AI assay. Apart from being based on a xanthone nucleus, these four compounds have several structural features in common. Of the 12 xanthones tested, 3, 4, 8
, and 9
are the only substances to bear hydroxy groups at C-1, C-3, and C-6, a prenyl group at C-2, and a 5 carbon substituent at C-8. Furthermore, in the cell-based AI assay, hydroxylation at C-7 (compounds 9
) was shown to elicit more aromatase inhibition than C-7 methoxylation (compounds 3
) (), with methoxylation leading to higher cytotoxicity than hydroxylation. Xanthones have only recently been tested for their ability to inhibit aromatase, with several synthetic xanthones exhibiting activity in the nanomolar range,23,24
but xanthones have not yet undergone extensive evaluation using additional in vitro, in vivo, or preclinical models. Further comparisons of the structure-activity relationship with both natural and synthetic xanthones should help to identify potential lead candidates for future development.
Xanthones are well known for their numerous and varied pharmacological effects, including having antioxidant, antimicrobial, central nervous system (CNS) depressant or stimulant, antihypertensive, antidiabetic, anticancer, anti-inflammatory, hepatoprotective, and/or immunomodulation properties.23
Xanthones used during the present study were isolated from Garcinia mangostana
L. (mangosteen), a slow-growing tropical tree with edible fruits.21
Mangosteen is commonly refered to as the “queen of fruits,” prized for its delicious fruits and has been utilized in Southeast Asian traditional medicine for stomach ailments (pain, diarrhea, dysentery, ulcers), as well as to treat infections and wounds.25,26
Owing to their activity as potent antioxidants,21,27–29
some mangosteen-based botanical products are standardized to contain high levels of xanthones such as α- and γ-mangostin. Mangosteen products have recently become one of the top-selling botanical dietary supplements in the U.S., and in 2005 represented the sixth-ranked single-herb dietary supplement with sales of over 120 million dollars, a substantial increase over the previous year.30
The results from the present study, including the testing of both mangosteen extracts and compounds, indicate that certain xanthones from mangosteen fruits act as potent aromatase inhibitors in both noncellular and cell-based AI assays, especially γ-mangostin (9
). Approximately two-thirds of post-menopausal women with breast cancer have the estrogen-dependent (hormone-dependent) form of this disease, in which estrogen is required for the growth of tumors.31
Because of their relatively high yield of xanthones such as α- and γ-mangostin (8
) in the pericarp of G.mangostana
mangosteen botanical dietary supplements may be acting as aromatase inhibitors, and might thus have a potential role in cancer chemoprevention for postmenopausal women with hormone-dependent breast cancer. However, before a definitive role of the mangosteen xanthones in this regard can be ascertained, additional work will need to be performed, including evaluation of the in vitro aromatase inhibitory xanthone constituents in an appropriate in vivo model.